The lead pipe is one of the more common weapons. It comes with 20 swings of the weapon before it breaks. You simply have to defeat 100 enemies across your playthroughs while this weapon is in your hand. Given it's length if you line enemies up you can achieve multiple K.O's with a single swing. Be careful however as even if you hit nothing the swings will still cost you. This trophy will pop mid-stage upon your 100th K.O, swing away George!
EDIT: Upon playing through the campaign further I've noticed that the North Area Nature Park stage is great for racking up Lead Pipe kills. At the start if you turn around there is one in a crate, and many more throughout the stage in both boxes as well as dropped by the bears along the way.